1. Field of the Invention
The Invention is a wire apparatus for use in precisely measuring the pitch diameter of an external thread, such as a bolt or other threaded fastener. The wire apparatus is configured to allow gauge wire to contact the external thread at three locations. A person may hold the external thread in one hand and precisely measure the pitch diameter using a micrometer, caliper or other measuring device held in the person's other hand.
2. Description of the Prior Art
The accurate measurement of external threads, such as the threads appearing on a bolt, is a necessary part of fastener manufacture and also a necessary aspect of quality control for any operation involving assembly using threaded fasteners. A critical dimension of a threaded fastener is the pitch diameter. The “pitch diameter” for a straight thread is the diameter of an imaginary cylinder axially aligned with the threaded fastener and located where the width of the thread and the width of the space between adjacent threads are equal.
Several methods to measure pitch diameter are in use, including the use of three wires. To use the three-wire method, three wires of equal diameter are held against the external thread by a fixture. The diameter of the wires is selected to engage the thread at the expected pitch diameter of the external thread. The distance between the outside edges of the wires then is measured. The wires generally are held by table top or bench top-mounted fixtures that align the wires with the external thread and that hold the wires in engagement with the external thread. The fixtures generally include a micrometer mounted normal to the axis of the external thread for measuring the distance between the outside edges of the wires. Well-established mathematical formulae are used to determine the pitch diameter from the measurements thus obtained.
The three-wire method is frequently used as a precision measuring technique to check the accuracy of plug gauges or other reference thread gauges.
None of the prior art thread gauging methods includes configuring the wire to hold the three wires in engagement with the external thread without use of a fixture.